The present invention relates in general to power combiners/dividers. More specifically, the invention relates to power combiners/dividers of a coupled transmission line (quarter-wavelength) type that enables significant increases in operating bandwidth.
Power combiners/dividers are essential subsystems in modem communication, HDTV and other systems, and play a major role in solid-state power amplifiers to achieve the specific output power. The necessary bandwidth of systems is permanently increasing, but on the other side the insertion loss and cost of power combiners should be minimized. There are two principal different technologies, which currently provide broadband power combining/dividing with isolation between ports, namely, transformer-type devices, usually with ferrite cores, to realize multi-octave bandwidth by providing RF isolation of their main operating conductors from ground, and quarter-wavelength (or multiple quarter-wavelength) devices without ferrite materials, where common ground is one of the operating conductors. The latter category of power combiners/dividers has, practically, significantly less bandwidth due to resonance properties of lines. On the other hand, these devices in most cases are much better for implementation in VHF-UHF bands and extension of their operating bandwidth remains still the open problem.
There are several main parameters that should be achieved simultaneously in broad band: low inputs/output voltage standing wave ratio (VSWR), high isolation between ports, small magnitude and phase unbalance in transfer characteristics, low insertion loss, acceptable complexity and size, high reliability and low cost. One example of a known power combiner/divider is the Wilkinson power divider (See, E. J. Wilkinson, xe2x80x9cAn N-Way Hybrid Power Dividerxe2x80x9d, IRE Transaction on Microwave Theory Tech., vol. MTT-8, pp. 116-118, January 1960; and S. Y. London, xe2x80x9cIndependent Operation of High Power VHF-Amplifiers on Common Loadxe2x80x9d, Problems of Radio-Electronics, ser. 10, vol. 6, pp. 87-97, 1959, USSR). This device provides N-way equal power combining or dividing at relatively low bandwidth of about one octave. A known way of extending bandwidth is to increase the number of sections in combiner/divider (See, Harlan Howe, J. R.: xe2x80x9cStripline Circuit Designxe2x80x9d, Artech House, Inc., 1974, Ch. 3). For an N-way M-section power combiner/divider, Nxc3x97M transmission lines and Nxc3x97M isolating resistors if N greater than 2 and M resistors for N=2 in the common case.
In cases when N greater than 3 and M greater than 2 (for achieving broad band) the real design becomes very complicated. Further, for M greater than 1 the isolating resistors have non-standard and different values of resistance in sections. In addition, for this type of power combiners the isolating resistors are xe2x80x9cfloatingxe2x80x9d and connected directly to the xe2x80x9cbodyxe2x80x9d of combiner. The latest disadvantage can be excluded by using additional transmission lines in various configurations (See, S. Y. London: xe2x80x9cPower Combiner of Several Amplifiersxe2x80x9d, USSR Patent No. 132674, 1960; U. H. Gysel: xe2x80x9cA New N-Way power Divider/Combiner Suitable for High-Power Applicationsxe2x80x9d, MIT Symposium Digest, 1975 pp. 116-118; T. I. Frederick et al., xe2x80x9cHigh Power Radio Frequency Divider/Combinerxe2x80x9d, U.S. Pat. No. 5,455,546; R. J. Blum, xe2x80x9cMicrowave High Power Combiner/Dividerxe2x80x9d, U.S. Pat. No. 5,410,281. However, such improvements are practically reasonable only for one-section combiners/dividers with relatively low bandwidth of about one octave.
Operating bandwidth of the above-described in-phase power combiners may be increased up to two octaves by using additional LC-correction elements, as has been shown by Arie Shor: xe2x80x9cBroadbanding Techniques for TEM N-Way Power Divider,xe2x80x9d 1988 MTT-S Digest pp. 657-659. However, this way of extending bandwidth implies increasing insertion losses and complexity.
One effective way to increase bandwidth of considered in-phase power combiners is to use coupled transmission lines (See, Europaische Patentaneldung, No. 0 344 458 A1, 1989). In U.S. Pat. No. 5,543,762, a simple one-section coupled-line structure is described in which the achieved bandwidth is less than two octaves for any built-in impedance transformation ratio in the combiner. However, if the required bandwidth is two octaves or more, it is impossible to realize acceptable isolation between ports as well as impedance transformation in known one-section structure, and a very complicate power combiner should be used independent on value of built-in impedance transformation ratio.
In view of the above, it is an object of the present invention to provide a broadband power combiner.
It is another object of the present invention to provide one-section N-Way power combiner with high isolation between its N outputs at two and more octave bandwidth.
It is still another object of present invention to provide power combiner having high isolation between ports by using only one group of isolating resistors.
It is still another object of present invention to provide power combiner having low inputs and output voltage standing wave ratio.
It is a further object of the present invention to provide N-Way power combiner having a symmetrical configuration with respect to its inputs to avoid phase and amplitude imbalances.
It is a further object of the present invention to provide a power combiner using well-known technology.
It is still a further object of the present invention to provide a power combiner using standard values of isolating resistance, 50-Ohm in the case of 50-Ohm nominal input impedance of the power combiner.
It is a still further object of the present invention to provide an N-way power combiner having broad bandwidth and built-in impedance transformation using a small number of stages.
In the present invention, significant effect in extending bandwidth or in simplifying multi-octave power combiner may be attained if functions of isolation between ports and impedance transformation (when necessary) are separate, i.e. a power combiner with full built-in impedance transformation is not used. A high isolation between ports in the bandwidth up to three octaves can be achieved in a simple onesection N-Way power combiner with only one group of N isolating resistors. Then the additional impedance transformer at the output of combiner should be used when necessary. This transformer will be much simpler than realization of built-in transformation in multi-section combiner because there are no specific restrictions on its structure and element values. Not only stepped quarter-wavelength structure may be used. Further, in a two-section power combiner in accordance to present invention a decade and more bandwidth may be achieved. In a more limited bandwidth a full built-in impedance transformation also may be implemented.